Fix hg:// and darcs:// dev url handling
Fix hg:// and darcs:// dev url handling

These are not real transports, the prefix needs to be stripped.
We default to http:// underneath, also allowing the ugly 'hg://http://xxx' for compat
with older opam that won't allow 'hg+http://' (it won't work for those, but they won't
complain, at least).

abate/opam,abate/opam